1. Field of the Invention
This invention relates to means for transmitting brake force between a brake activating key and a brake shoe in a drum brake, said means comprising first and second interengaging, threaded elements a relative rotation of which will cause variation of their total axial length, the remote end surfaces of said means having curved surfaces engaging surfaces on said key and said brake shoe.
2. Description of the Prior Art
The purpose of providing means having variable total length is to be able to compensate for brake shoe lining wear and drum brake wear, thus maintaining the angular movements of the key during braking independent of the wear of the braking surfaces on the drum and the brake lining. The relative rotation of the two elements of the means may be caused manually, but advantageously by an automatic slack adjusting device.
However, the use of a compression force transmitting element between the brake key and the brake shoe has hitherto caused certain problems in the brake due to the use of axially movable brake shoes--i.e. brake shoes movable in the direction of the axis of the brake drum--and due to the use of spherically shaped surfaces for the force transmission between the key and the brake force transmission means as well as between said means and the brake shoe.
An axial displacement of the brake shoe will involve a greater angular displacement of the brake key in order to activate the brake and this will in turn involve a longer piston-stroke of the pneumatic piston-cylinder brake activator usually used in the larger type of vehicles generally using this type of brake. A longer piston stroke may result in insufficient brake force and in insufficient brake activating time.
In case the brake is equipped with an automatic slack adjusting device for performing the above mentioned relative angular displacement of the two elements of the brake force transmission means, an axial displacement of the brake shoe will cause an increase of the piston stroke necessary for application of the brake and the adjuster will function as if the braking surfaces had been exposed to wear i.e. the automatic slack adjuster will reduce the slack between the braking surfaces and said slack may even become negative making it impossible to release the brake.